1. Field of the Invention
The present invention relates to a variable gain amplifier, and more particularly, it relates to a variable gain amplifier widening its input range.
2. Description of the Background Art
In a transmitter/receiver for radio communication such as mobile communication, the signal strength may extremely vary with environmental factors. In order to more correctly transmit/receive signals, power control is necessary for increasing or reducing the gain of an amplifier if the signal strength is small or large. A variable gain amplifier capable of varying its gain with the signal strength is employed for such power control.
&lt;Device Structure&gt;
FIG. 13 shows the structure of a conventional variable gain amplifier 90. As shown in FIG. 13, the variable gain amplifier 90 comprises a high-gain differential amplifier 1 and a low-gain differential amplifier 2 having common resistances (load resistances) R1 and R2. The resistances R1 and R2 have the same resistance value RL.
The differential amplifier 1 has NPN transistors Q1 and Q2 whose collector electrodes are connected to the resistances R1 and R2, having the resistance value RL, which are connected to a power source Vcc in common. The emitter electrodes of the transistors Q1 and Q2 are connected to the collector electrodes of NPN transistors Q3 and Q4 respectively, and connected in common with each other through a resistance (feedback resistance) R3. The emitter electrode of the transistor Q3 is connected to the collector electrode of an NPN transistor Q5, whose emitter electrode is connected to a ground level GND through a resistance R4.
The differential amplifier 2 has NPN transistors Q6 and Q7 whose collector electrodes are connected to the resistances R1 and R2 which are connected to the power source Vcc in common. The emitter electrodes of the transistors Q6 and Q7 are connected to the collector electrodes of NPN transistors Q8 and Q9 respectively, and connected in common with each other through a resistance (feedback resistance) R5. The emitter electrode of the transistor Q9 is connected to the collector electrode of an NPN transistor Q10, whose emitter electrode is connected to the ground level GND through a resistance R6.
The base electrodes of the transistors Q1 and Q6 are connected to an input terminal T1, those of the transistors Q2 and Q6 are connected to an input terminal T2, those of the transistors Q3 and Q4 are connected to a control terminal T3, those of the transistors Q8 and Q9 are connected to a control terminal T4, those of the transistors Q5 and Q10 are connected to a variable bias input terminal T5, the collector electrodes of the transistors Q2 and Q7 are connected to an output terminal T6, and those of the transistors Q1 and Q6 are connected to an output terminal T7.
Further, the emitter electrodes of the transistors Q8 and Q4 are connected to the collector electrodes of the transistors Q5 and Q10 respectively.
A circuit formed by the transistors Q3, Q4, Q8 and Q9 is referred to as a control circuit 3, and that formed by the transistors Q5 and Q10 and the resistances R4 and R6 is referred to as a variable current source 4.
The resistances R3 and R5, which are feedback resistances, have values REG and REL respectively, in the relation REG&lt;REL.
In general, the gain variability with respect to a control voltage and an input range characteristic indicating an allowable input range causing no output distortion are regarded as the important performance of a variable gain amplifier. The gain variability and the input range characteristic of the variable gain amplifier 90 are now described.
&lt;Gain Variability&gt;
The gain variability is now described. The respective gains G0G and G0L of the differential amplifiers 1 and 2 and the total gain G0T of the variable gain amplifier 90 can be expressed in the following numerical formulas (1), (2) and (3) respectively: ##EQU1##
Referring to each of the numerical formulas (1) to (3), VT represents a thermal voltage, which is expressed in kT/q, where k represents the Boltzmann's constant, T represents the temperature, and q represents the elementary charge quantity. A control voltage Vct supplied between the control terminals T3 and T4 controls operating currents IQ0G and IQ0L of the differential amplifiers 1 and 2.
FIG. 14 shows exemplary change states of the operating current IQ0G (the total of currents flowing through the transistors Q1 and Q2) and the operating current IQ0L (the total of currents flowing through the transistors Q6 and Q7) of the differential amplifiers 1 and 2 with respect to change of the control voltage Vct. When the control voltage Vct shown on the horizontal axis of FIG. 14 is varied in the range of .+-.0.1 V, the operating currents IQ0G and IQ0L shown on the vertical axis vary in the range of substantially zero to the bias current IEE (the total of currents flowing through the resistances R4 and R6) of the differential amplifiers 1 and 2 respectively.
When the control voltage Vct is gradually reduced from such a state that most part of a current of a constant current source 4 flows to the differential amplifier 1, for example, the operating currents IQ0G and IQ0L reduce and increase respectively. Thus, it is understood that the gains G0G and G0L of the differential amplifiers 1 and 2 reduce and increase respectively from the numerical formulas (1) and (2). If an input signal is extremely large, therefore, it is possible to prevent an internal device (not shown) which is connected with the output terminals T6 and T7 from breaking due to excessive amplification of the input signal by reducing and increasing the gains G0G and G0L of the differential amplifiers 1 and 2 respectively. If the input signal is extremely small, on the other hand, it is possible to amplify the same to a level suitable for the internal device (not shown) by increasing the gain G0G of the differential amplifier 1.
Thus, the variable gain amplifier 90 comprising the high-gain differential amplifier 1 and the low-gain differential amplifier 2 can cope with an extremely large or small input signal by adjusting the control voltage Vct. In this case, however, the variable gain amplifier 90 simply adds up the currents flowing to the high-gain differential amplifier 1 and the low-gain differential amplifier 2 and feeds the total current to the resistances R1 and R2. If the difference between the gains G0G and G0L of the differential amplifiers 1 and 2 is remarkable, therefore, the gain G0G of the differential amplifier 1 is dominant. This phenomenon is now described with reference to FIG. 15.
Referring to FIG. 15, the horizontal and vertical axes show the control voltage Vct and the gains respectively. Symbols G0G, G0L and G0T denote the gain characteristics of the differential amplifiers 1 and 2 and the overall variable gain amplifier 90 respectively. It is understood from FIG. 15 that the gain of the differential amplifier 1 becomes dominant from the value of the control voltage Vct at which the gain characteristics G0G and G0L intersect with each other, such that the gain characteristic G0T substantially coincides with the gain characteristic G0G. Namely, the term VT/IQ0G is dominant in the numerical formula (1) and the gain characteristic G0G remarkably increases when the operating current IQ0G is small. On the other hand, the operating current IQ0L is large and decided by the resistance value REL in the numerical formula (2), and hence the gain characteristic G0L remains substantially unchanged. If the gain characteristic G0G exceeds the gain characteristic G0L, therefore, the gain characteristic G0G becomes dominant in the gain characteristic G0T, as understood from the numerical formula (3). If the operating current IQ0G increases, further, the operating current IQ0L reduces and the term VT/IQ0L becomes more dominant than the resistance value REL and the gain characteristic G0L reduces, to result in continuous domination of the gain characteristic G0G.
&lt;Input Range Characteristic&gt;
The input range characteristic is now described. The input range of a differential amplifier is generally decided by the product of the operating current and the feedback resistance. If the operating currents IQ0G and IQ0L are identical to each other, therefore, the differential amplifiers 1 and 2 have narrow and wide input ranges respectively due to the relation REG&lt;REL between the resistance values REG and REL of the resistances R3 and R5 which are emitter feedback resistances. This is because the base-to-emitter voltages of the transistors Q1 and Q2 increase due to a small voltage drop in the resistance R3 and base-to-emitter voltages of the transistors Q6 and Q7 reduce due to a small voltage drop in the resistance R5.
In a region of the control voltage Vct where the operating current IQ0G of the differential amplifier 1 is sufficiently close to zero, e.g., in the region of -0.1 V to -0.05 V shown in FIG. 14, the input range of the differential amplifier 2 is so sufficiently wide that the differential amplifier 2 substantially decides the input range of the variable gain amplifier 90. If the control voltage Vct increases beyond this range, however, the input range of the differential amplifier 2 narrows while that of the differential amplifier 1 so widens that the differential amplifier 1 substantially decides the input range of the variable gain amplifier 90. However, the input range of the differential amplifier 1 is so set that the maximum value thereof is smaller than that of the input range of the differential amplifier 2 in order to attain higher gain than the differential amplifier 2, whereby the input range of the variable gain amplifier 90 reduces as a result.
Due to the aforementioned structure of the conventional variable gain amplifier 90, the operating current IQ0L of the differential amplifier 2 abruptly reduces following positive increase of the control voltage Vct also in the region of the control voltage Vct where the differential amplifier 2 is dominant, e.g., in the range of -0.1 V to -0.05 V shown in FIG. 14 and the voltage applied to the resistance R5 reduces, and hence the input range disadvantageously abruptly reduces.
Further, the transistors are cascode (cascaded triode)-connected with each other in three stages as in the case of the transistors Q1, Q3 and Q5, and hence the maximum permissible input amplitude of each transistor reduces. Assuming that the base-to-emitter voltage in operation of each transistor is 0.8 V and the power supply voltage is 3 V, for example, the maximum permissible input amplitude of the base electrode is about 2.2 V (3 V-0.8 V). When the transistors are cascode-connected with each other in three stages, however, the maximum permissible input amplitude of the base electrode is about 0.6 V from 3 V-(0.8.times.3) V. The maximum permissible input amplitude of each transistor is preferably at a high level since the transistor output is distorted if an input voltage of the base electrode exceeds this value. In general, the input range of a differential amplifier is less than the maximum permissible input amplitude of transistors forming its differential pair, and hence the input range cannot be increased if the maximum permissible input amplitude of the transistors is small.